The thermal drawing technique is the main technique that is used to fabricate optical fibers, see reference [1].
In the drawing process of the prior art, a large preform made of glass or polymer is fed into an enclosed furnace and heated over its glass transition temperature. As the viscosity decreases several orders of magnitude, the preform necks down under its own weight; when the lower end of the preform comes out of the furnace, it is attached to a pulling system, and the fiber is then continuously drawn. Fiber dimension, shape and internal stress are monitored during the process by optical sensor and tension sensor; and they are controlled by a set of drawing parameters, namely feeding speed (the speed at which the preform is fed into the furnace), drawing speed (the speed at which the fiber is pulled) and the furnace temperature. The principle of this technology is illustrated in FIG. 1.
Since the invention of low-loss optical fiber by Charles Kao see reference [1], which redefined the way of communication, the thermal drawing process has been a subject for intensive research and has become extremely efficient in term of scaling-down ratio and ease of processing. New generations of fibers have emerged, such as photonic crystal fibers see reference [2] or multimaterial fibers see reference [3], bringing thermally drawn fibers to a wide range of application from optics and electronics to microfluidics and bioengineering.
An important aspect of the thermal drawing process is that it inherently generates a surface area as the fiber is being stretched. This simple and low-cost processing approach is therefore an ideal way to create micro/nano textured surfaces over large and curved area, which is a key attribute in many fields of applied science such as energy storage and harvesting, health care, smart textiles or distributed sensing. This aspect, however, has surprisingly received few attentions. Recently, Banaei and Abrouraddy see reference [4] proposed a design in which the outside surface of a step-index optical fiber played a role for solar concentrator, with the curvature of the outside structure being of several hundreds of micrometer; while Yildirim et al see reference [5] created star-shape fiber with feature size as small as 30 microns, starting from a lathe-shaping 3 cm circular preform.
In both works, preforms were prepared by mechanical machining from a single material (polycarbonate in reference [4] and polyethylenimine in reference [5]), and the shape preservation was due solely to the intrinsic properties of the constituting polymers, based on previous observation.
However, the resulting application only comes from the texturing of the polymer, and not from functional properties that other polymers or polymer composites could exhibit. Moreover, to create even smaller structure (for example a sub-micron structure) on various polymers, the single-material preform-to-fiber drawing approach represents several limitations. Firstly, since fiber drawing requires annealing the preform above the glass transition temperature of the polymer constituents, this heating also results in a thermal reflow and smoothing of structured surface, and the smoothing happens faster as the structure's feature size gets smaller.
Secondly, even if the shape remains, making smaller feature-size structure on fiber from millimeter-size structure on preform could only be achieved by high draw-down-ratio, which either results in very small fibers that would be too weak to be drawn or too small for some applications, or requires a very large, sometimes impractical initial preform.